Flame retardant polymethyl methacrylate compositions

ABSTRACT

Self-extinguishing homo- and copolymers of methyl methacrylate (MMA) are prepared by the intimate admixture of the latter polymers with a specified 2,3-dibromopropyl acid phosphate ester. The thus treated polymers display a high degree of flame retardance without any diminution of their clarity or hardness and they may be safely employed in any application requiring their possible exposure to fire.

United States Patent Weil [451 May 30, 1972 s41 FLAME RETARDANTPOLYME'IHYL 3,456,022 7/1969 Wright ..260/648 METHACRYLATECOL/[POSITIONS 3,294,730 12/1966 Jukes ...260/30.6 3,445,404 5/1969Ronden et al.. .....260/2.5 [72] Inventor: Edw r D- W il, ing nn.3,283,013 11/1966 Rimmer .260/633 NY. 3,030,327 4/1962 l-losch...260/30.6 [73] Assign: Sumner Chemical Company, New York, 3,001,9549/1961 Buchholz et a1. ..260/2.5

' NY Primary Examiner-Donald E. Czaja [22] Filed: Apr. 6, 1970 AssistantExaminer-R. A. White [211 pp No 26 050 Attorney-Wayne C. Jaeschke andMartin Goldwasser Related US. Application Data [57] ABSTRACT [63]Continuation-impart of Ser. NO. 463, Jan. 2, 1970, Self-extinguishinghomoand copolymers ofmethylmethacryabandone late (MMA) are prepared bythe intimate admixture of the latter polymers with a specified2,3-dibromopropyl acid [52] u.s.c1 ..260/45.7 P, 106/15 FP,252/8.lphosphate esterthus treated P y p y a high [51] lm. Cl. ..C09k 3/28degree of flame retardance Without y diminution of their 58 Field ofSearch ..260/45.7 P, 933, 963 clarity or hardness and they y be Safelyemployed in y pplication requiring their possible exposure to fire, [56]References Cited 5 Claims, No Drawings FLAME RETARDANT POLYMETHYLME'II-IACRYLATE COMPOSITIONS RELATED APPLICATION This application is acontinuation-in-part of application Ser. No.-463, filed Jan. 2, 1070,now abandoned.

BACKGROUND OF THE INVENTION Polymethyl methacrylate (PMMA) andcopolymers of MMA with minor proportions of one or more ethylenicallyunsaturated monomers are hard, optically clear, thermoplastic polymerswhich are widely utilized for the preparation of such diverse articlesas lenses, aircraft canopies, windows, Windshields, lighting fixturesand advertising displays. As normally prepared, these polymers arematerials which will ignite and continue to burn upon exposure toflames. However, in many instances, particularly where they are beingconsidered for use in building interiors, it is highly desirable thatthese homoand copolyrners of MMA should display flame retardantproperties so that they may conform to various building codes whichrequire a substantial degree of fire or flame retardancy for interiorbuilding materials.

Prior attempts to provide flame retardant homoand copolymers of MMA haveinvolved the use of a variety of additives including certain tris(halogenated alkyl) phosphates or phosphonates. However, the effectiveutilization of these phosphate esters required their presence in ratherhigh concentrations which adversely affected the physical properties ofthe treated polymers. Thus, the clarity and especially the hardness ofthe polymers were particularly prone to deterioration in the presence ofthe high concentrations of these phosphates or phosphonates which werenecessary to achieve a self-extinguishing polymer composition.

It is, therefore, the prime object of this invention to provide a meansof preparing flame retardant polymers of MMA which will not adverselyaffect their inherent hardness and clarity. Various other objects andadvantages of this invention will be apparent from the disclosurethereof which follows hereinafter.

TECHNICAL DISCLOSURE OF THE INVENTION Br r bis 2,3-dibromopropyl) acidpyrophosphate, i.e.,

if u CH -OHCHzOZ-OPO-GHg-CH-CH 1 31' Br H I r Br and 2,3-dibromopropylmethyl acid phosphate, i.e.,

i CH OIE-O-OH ICH(IJHQ OH Br Br and mixtures of any two or more of thelatter esters. It is to be noted that 2,3-dibromopropyl methyl acidphosphate is a new composition of matter.

Also applicable for use in the process of this invention are mixturescontaining at least about 30 mole percent of one or more of the abovelisted 2,3-dibromopropyl acid phosphate esters in combination with oneor more phosphate esters which, if used alone, would be inoperative asflame retardant additives for homo-and copolyrners of MMA. For example,mono-2,3-dibromopropyl acid phosphate is often prepared as part of amixture with di-( 2,3-dibromopropyl) acid phosphate and such mixturesare suitable for use in the process of this invention provided that themono ester is present in said mixtures in a concentration of at leastabout 30 mole percent. The product of 3 molar equivalents of2,3-dibromopropanol with 1 molar equivalent of phosphorus pentoxide,said product containing at least about 30 mole percent of mono-2,3-dibromopropyl acid phosphate together with bis-( 2,3- dibromopropyl)acid phosphate is also useful in the process of the invention. However,the mono-2,3-dibromopropyl acid phosphate is far more efficacious whenused in mixtures where it is present in a concentration greater thanabout 30 mole percent.

Still more effective, and having the important advantage of lower color,is the product obtained by reacting (3-X molar equivalents of2,3-dibromopropanol and X molar equivalents of methanol with 1 molarequivalent of phosphorus pentoxide, where X has a value of from about0.1 to about 1.6). These beneficial effects, with respect to reducedcoloration and enhanced flame retardance, which are attained byreplacing some of the dibromopropyl groups by methyl groups in thesephosphate acid esters is surprising and unexpected and no explanationcan be offered for this phenomenon. Optimum results, with respect tothis group of mixed methyl dibromopropyl phosphates is the mixturecontaining at least about 30 mole percent of 2,3-dibromopropyl methylacid phosphate with mono-2,3-dibromopropyl acid phosphate as made byaddition of 1 molar equivalent of methanol to 1 molar equivalent ofbis-2,3-dibrornopropyl acid pyrophosphate.

The above listed phosphate esters may be prepared by means of reactionsknown to those skilled in the art. Thus, bis(2,3-dibromopropyl) acidpyrophosphate can be made by the reaction of phosphorus pentoxide with2,3- dibromopropanol; while a mixture of mono-2,3- dibromopropyl acidphosphate and 2,3-dibromopropyl methyl acid phosphate can be made byreacting methanol with bis( 2,3-dibromopropyl) acid pyrophosphate.

It is quite surprising to find that such closely related phosphateesters, with respect to the above specified group of compounds, asbis-(dibromopropyl) acid phosphate; tris-( 2,3- dibromopropyl)phosphate; and, mixtures of monoand di- (2,3-dibromopropyl) acidphosphate wherein the di-ester is present in a concentration greaterthan 30 mole percent are found to be unsatisfactory as flame retardantadditives for MMA polymers. Thus the effective use of the lattercompounds requires their presence in high concentrations which adverselyaffect the clarity and hardness of the thus treated PMMA.

In utilizing the above described phosphate esters as flame retardantadditives for MMA polymers, they may be incorporated directly into thepolymerization recipe, i.e., the mixture of a catalyst with methylmethacrylate monomer along with any other desired vinyl monomers, thatis utilized for the preparation of the MMA polymer or copolymer by meansof a bulk, or solvent free, polymerization process. Applicable catalystsinclude azobisisobutyronitrile, benzoyl peroxide and lauroyl peroxide.

Alternatively, the selected phosphate ester may be intimately admixedwith a previously prepared, partially polymerized syrup" of MA or of amixture of MA with one or more vinyl comonomers prior to the casting ofthe syrup. These syrups comprise an MMA homoor copolymer dissolved inthe unpolymerized MMA or in the unpolymerized mixture of MMA with one ormore vinyl comonomers of which said polymer is composed. The thustreated syrup is then cast into a mold where its polymerization iscompleted. in addition, the phosphate ester additive may be admixed witha solid sample of the MMA polymer at such time as the latter is to besubjected to a milling, extruding, calendering, molding or otherconverting operation prior to its being put into the form or shape inwhich it will ultimately be utilized.

With respect to proportions, the phosphate ester may be present in thefinal MMA polymer-phosphate ester blend in a concentration of from about2 to 30 percent, as based on weight of the polymer, the exact leveldepending on the degree of flame retardancy required. Within the latterlimits, the resulting product will display excellent flame retardantproperties without serious reduction in either the clarity or hardnessof the polymer. Optimum results are, however, attained by the use of thephosphate ester in a concentration of about 5-25 percent as based on theweight of the MMA polymer.

ln addition to PMMA, the above specified phosphate esters may also beused to provide flame retardant properties for copolymers of methylmethacrylate with minor proportions of one or more alpha,beta-ethylenically unsaturated monomers including: the C,C alkyl,cycloalkyl and bicycloalkyl esters of acrylic acid and the C -C alkyl,cycloalkyl and bicycloalkyl esters of methacrylic acid such, forexample, as ethyl acrylate and methacrylate, butyl methacrylate,ethylhexyl methacrylate, norbornyl acrylate, and cyclohexyl acrylate;vinyl aryl compounds such, for example, as alpha-methyl styrene andstyrene; and, nitriles of alpha, beta-ethylenically unsaturatedcarboxylic acids such, for example, as acrylonitrile, methacrylonitrileand alpha-chloroacrylonitrile. From the above given group, the use ofthe C,C alkyl esters of acrylic acid and of the C -C alkyl esters ofmethacrylic acid is preferred.

Moreover, the PMMA-phosphate ester blends of this inven tion may, ifdesired, include various optional additives therein including, forexample, ultra-violet screening agents such as hydroxybenzophenones;mold release agents such as stearic acid; and, plasticizers such as thedialkyl phthalate and the aryl phosphate esters in order to lower heatdistortion temperatures. The compositions may also contain pigments,dyes, opacifying agents, decorative additives such as reflective metalfoils or flakes, and other imbedded solid objects such as glass cloth orfibers, paper, and the like. In addition, the compositions may containother flame retardants such as halogenated alkyl phosphates orphosphonates, alkyl acid phosphates, or small concentration ofphosphoric acid.

The flame retardant homoand copolymers of MMA resulting from the processof this invention may be put into the form of panels, sheets, rods,tubes, films and shaped articles, etc. by means of converting procedureswell known to those skilled in the art. They are particularly useful ininterior applications requiring the use of fire resistant materialssuch, for example, as lighting fixtures wherein the normal clarity andhardness of the MMA polymer must be retained.

In addition to being blended with homoand copolymers of MMA, the acidphosphate compositions of the invention can also be utilized as flameretardant additives for various other thermoplastic and thermosetpolymers, elastomers, and other combustible substrates. For example,these compositions are useful as flame retardant additives forpolyesters, both cured and uncured; polystyrene; polyolefins;acrylonitrile-styrenebutadiene copolymers and other styrene copolymersand polyblends; natural and synthetic rubber; phenolic resins;urea/formaldehyde resins; melamine/formaldehyde resins; epoxy resins,diallyl phthalate resins; vinyl resins; polyarylene oxides; polyamides;polycarbonates; polysulfones; and cellulosics such as paper, wood,fiberboard, cellulose acetate, cotton, rayon, and the like.

in the phenolic, epoxy, melamine/formaldehyde, and urea/formaldehyderesins the acid phosphate compositions of this invention may serve inthe double role of flame retardant additives and as acid catalystsand/or reactants to aid in the curing of the resin. In systems where anacidic material may be objectionable, the acid phosphates may beneutralized by a nitrogen base, such as ammonia, an amine, melamine orbasic derivative thereof or urea or a basic derivative thereof,generally without loss of flame retardant activity or even withenhancement thereof. Such neutralized compositions are especiallysuitable for cellulosic substrates. In some systems such as rubber, theacid phosphates may be rendered neutral by salt formation with antimony,calcium, zinc, lead or other metals, conveniently introduced into thesystem as the oxide or carbonate.

Although the methyl ester is the preferred embodiment of the invention,other lower alkyl (C -C dibromopropyl acid phosphates are workable, forexample ethyl, propyl, and butyl 2,3-dibromopropyl acid phosphates arefound to be effective in the process and products of the invention.

The following examples will further illustrate the embodiment of thisinvention. In these examples, all parts given are by weight unlessotherwise noted.

EXAMPLE I This example illustrates the preparation of a flame retardantPMMA composition by means of the process of this invention. it alsoprovides a comparison with the results obtained with phosphate estersother than those specified as applicable for use in this process. I

A number of difierent batches of PMMA were polymerized, by means of aconventional bulk polymerization procedure, in the presence,respectively, of the following phosphate esters: l)mono-2,3-dibromopropyl acid phosphate; (2) a mixture containing monoanddi- (2,3-dibromopropyl) acid phosphate in a 1:1 molar ratio; and, (3)di-(2,3- dibromopropyl) acid phosphate, the latter ester beinginapplicable for use in the process of this invention. The bulkpolymerization procedure which was utilized involved dissolving thedesired concentration of the phosphate ester or ester mixture in methylmethacrylate monomer containing 100 PPM OF azobisisobutyronitriledissolved therein. The resulting solutions were poured into tubularmolds and then heated to a temperature of about C. The polymerizationwas allowed to proceed for a period of about 3 hours yielding solidpolymer rods having a diameter of 10mm. Each batch of the thuspolymerized PMMA rods contained its respective phosphate ester or estermixture in a concentration of 6 percent, as based on the weight of thePMMA.

It was noted that all of these rods displayed excellent hardness andclarity except that the rods containing the inapplicabledi-(2,3-dibromopropyl) acid phosphate had an orangebrown coloration. Theflame retardancy of these rods was then evaluated by igniting them,while in a horizontal position, with a gas burner and noting the amountof time required for the resulting flames to be extinguished. Thus, itwas observed that: (l) the PMMA rods containing 6 percent, by weight, ofmono-2,3-dibromopropyl acid phosphate were completely self-extinguishedwithin 10 seconds; (2) the PMMA rods containing 6 percent, by weight, ofa mixture containing monoand di(2,3-dibromopropyl) acid phosphate in a1:1 molar ratio were completely self-extinguished within 20 seconds;and, 3) the PMMA rods containing 6 percent, by weight, ofdi-(2,3-dibromopropyl) acid phosphate were not self-extinguishing.Rather, they continued to burn until they were completely consumed.

EXAMPLE II This example illustrates the preparation of bis (2,3-dibromopropyl) acid pyrophosphate which, in turn, was used for thepreparation of an equimolar mixture of mono- 2,3- dibromopropyl acidphosphate and 2,3-dibromopropyl methyl acid phosphate.

A reaction vessel containing 436 parts of 2,3- It dibromopropanol washeated to a temperature of 50 C. L01: (1051 -[-'[N X whereupon 141 partsof phosphorus pentoxide were added gradual y With st g- The resulting Xl' W35 Stirred for 2 Thus, a higher LOl is indicative of a higher degreeof flame rehours at 5560 C. until the solids had substantially disap- 5tardancy. peared. The bis (2,3-dibromopropyl) acid pyrophosphate thusThe following table presents the results of this evaluation. producedwas found by nuclear magnetic resonance to have a Formulation No. L0](3%) ratio of two acid protons per aliphatic CH protons.

After heating the latter reaction product to 5070 C., 32

parts of methanol were added with stirring. This mixture was 10 g heldat 70 C. for one-half hour. The nearly colorless syrup 3 26.7

thus produced was found by means of nuclear magnetic g resonance andtitration analysis to correspond to a 1:1 molar 6 mixture ofmono-2,3-dibromopropyl acid phosphate and 2,3- 7 20.0

dibromopropyl methyl acid phosphate. The latter esters were 3 thenseparated for purposes of characterization by column chromatography onsilica gel. The 2,3-dibromopropyl methyl 1] 2L0 7 acid phosphate' wa'sfound to be a colorless solid having a V 7 12 23.0

neutralization equivalent of 326, and the 2,3-dibromopropyl 2O acidphosphate was a colorless liquid having a neutralization Th above dclearly indicates that h PMMA blends equivalent 12 to the strong acidend P containing the phosphate esters designated as applicable for usein the process of this invention, i.e., Formulations Nos.

EXAMPLE m l-9 display substantially superior flame retardancy as com- 25pared with the PMMA formulations, i.e., Formulations Nos. This exampleillustrates the preparation of additional fire 2, which Contained aphosphate ester, namely retardant PMMA formulations according to theprocess of this d p py p p which is not within the g p of invention andalso offers. a comparison betwee th hu esters deemed suitable for use inthis process. It is to be noted prepared fonnulations and the resultsobtained by the use of a that the hardness and clar y of the rods m fromF rm lapresently available phosphate ester additive. tions Nos. l-9 werecomparable to that observed in a sample A number of PMMA-phosphate esterformulations were f nmo fi PMMA Shee ingprepared by means of a proceduresimilar to that of Example I,

hereinabove, except that a partially polymerized polymethyl EXAMPLE IVmehacrylate'methylmethacrylate y p was p y in This example illustratesthe application of the process of place of unreacted MMA monomer. As acuring catalyst, 200 i invention to a number f MMA copolymers ppmazobisisobutyronitrile were added and the formulations p A were heated,at 80 C., in tubular molds until hardened. The A 5 iL sheet f clear h -dpolymer was b i b h composition of the thus prepared rods is set forthbelow. ing h f ll i f ul ti at 30 c i a mold f a Formulation No. I--PMMAwith 5%, by weight, of an i d f 10 h equimolar mixture ofmono-2,3-dibromopropyl acid 40 phosphate and 2,3-dibromopropyl methylacid p phosphate; the latter phosphate ester mixture hereinaftermono-2,3-dibromopropyl acid phosphate 15 being referred to as MixtureA." z g pg 'ryig z 8 Formulation No. 2--PMMA with 13 percent, by weight,of benioyl peroxide Q02 Mixture A. Formulation 3'PMMA with 20%, yweight, of This sheet was found to be self'extinguishing when evalutureated by means of ASTM test D-635. Formulation No. 4--PMMA with 5percent, by weight, of p l Pl acid phosphate- A 5 mil. sheet of clearhard polymer was obtained by heat- Fmmulation 5*PMMA with 13 Percent, yg of ing the following formulation for 2 hours at 70 C. and i p py acidp phate. then for 5 hours at 100 c. while contained within a mold:Formulation No. 6PMMA with 20 percent, by weight, of

mono-2,3-dibromopropyl acid phosphate. Pans Formulation N0. 7PMMA with 5percent, by weight, of an equimolar mixture of 2,3-dibromopropylbis-(2,3-dibromopropyl) acid pyrophosphate. methyl acld PhSPhate anddlbromopropyl acid phosphate. 10 Formulation No. 8PMMA with 13 percent,by weight, of methyl methacrylate [00 bis-( 2,3-dibromopropyl) acidpyrophosphate. Z-ethylhexyl acrylate 5 Formulation No. 9PMMA with 20percent, by weight, of lauloyl Peroxide bis-( 2,3-dibromopropyl) acidpyrophosphate. 6O Formulation p with 5 percent, by weight, f The sheetwas found to be self-extinguishing when evalutris (2,3-dibromopropyl)phosphate; the latter ester being ated y means of ASTM test Similarresults were inapplicable for use in the process of this invention.obtained y Substituting cyclohexyl methactylate and Formulation No. 11-PMMA with 13 percent, by weight, ethylhexyl methacrylatei respectively,for the Zethylexyl of tris (2,3-dibromopropyl) phosphate. acrylate m theabove formulauon- Formulation No. 12-PMMA with 20 percent, by weight, of

tris 2,3dibromopropyl) phosphate. EXAMPLE v The tire retardancy of thesetubes was then quantitatively This example illustrates the preparationof additional evaluated by determining their Limiting Oxygen Index (L01)products by means of the process of this invention. by means of theprocedure described by Fenimore and Martin Part A in the November, 1966,issue of Modern Plastics. In brief, this To a mixture of 1.8 moles ofmethanol and 1.2 moles of procedure directly relates flame retardancy toa measurement redistilled 2,3-dibromopropanol, there was gradually addedof the minimum percentage concentration of oxygen in a oxwith stirring1.0 mole of phosphorus pentoxide; the addition of ygenznitrogen mixturewhich permits the sample to burn; the the latter reagent being completedover a period of 45 minutes LOI being calculated as follows: at 3040 C.The resulting mixture was then stirred at 70 C.

for 30 minutes so as to yield a mixed dibromopropyl methyl acidphosphate containing:

(a) at least about 30 mole percent of a mixture of mono-2,3-

dibromopropyl acid phosphate and 2,3-dibromopropyl methyl acid phosphatetogether with (b) a mixture of monoand dimethyl acid phosphates and bis(2,3-dibromopropyl) acid phosphate. This mixture was in the form of aclear, substantially colorless liquid when used as a flame retardantadditive for PMMA, the resulting plastic sheets were found to becolorless and were self-extinguishing in air at a concentration of 6.0percent as based on the weight of the PMMA.

Part B For comparative purposes, the above described reaction was run inthe absence of methanol and with 3.0 rather than 1.2 moles of2,3-dibromopropanol. This reaction yielded a mixture containing:

(a) at least about 50 mole percent of mono-2,3-

dibromopropyl acid phosphate together with (b) bis(2,3-dibromopropyl)acid phosphate. When used as a flame retardant additive for PMMA, theresulting plastic sheets were found to be colorless and wereself-extinguishing in air at a concentration of 6.0 percent, as based onthe weight of the PMMA.

Part C In an evaluation of comparative flame retardant properties, a LOIof 25.7 percent was achieved by the use of a concentration of 16.7percent, as based on the weight of PMMA, of an equimolar mixture ofmono-2,3-dibromopropyl acid phosphate and 2,3-dibromopropyl methyl acidphosphate. In contrast, this same level of flame retardancy could beachieved only when a concentration of 47.2 percent by weight of PMMA, ofdi-n-butyl acid phosphate or of 41.8 percent, by weight of PMMA, of tris(2,3-dibromopropyl) phosphate were utilized. However, it is to bepointed out that the use in the above specified concentrations of thelatter additives, neither of which is designated as being applicable foruse in the subject process, yielded PMMA compositions which were toosoft for commercial utilization.

EXAMPLE VI To 1.0 mole of methanol and 2.0 moles of 2,3- dibromopropanolthere was added 1.0 mole of phosphorus pentoxide. The admixture of thesereagents was conducted with cooling over a period of 40 minutes at atemperature in the range of from 1548 C. and the mixture was thenmaintained for 1 hour at 70 C. so as to obtain a quantitative yield of amixed reaction product.

Analysis of this mixed reaction product by means of solvent partitionanalysis involving water and benzene phases followed by nuclear magneticresonance analysis of the fractions thereby obtained revealed that itscomposition was:

Mole Mono-2,3-dibromopropyl acid phosphate 25-30 2,3-dibromopropylmethyl acid phosphate 25-30 Mono-methyl acid phosphate 20-25Bis(2,3-dibromopropyl) acid Phosphate 20-25 When used as a flameretardant additive for PMMA, the resulting plastic sheets were found tobe colorless and were self-extinguishing in air at a concentration of6.0 percent as based on the weight of the PMMA.

Variations may be made in proportions, procedures and materials withoutdeparting from the scope of this invention as 7 defined in the followingclaims.

What is claimed is:

1. A flame retardant composition comprising an intimate mixture of apolymer of methyl methacrylate and as a flame retardant additive amixture of mono-2,3-dibromopropyl acid phosphate and 2,3-dibromopropylmethyl acid phosphate.

2. A flame retardant composition comprising an intimate mixture of apolymer of methyl methacrylate and as a flame retardant additivebis(2,3-dibron 1opropyl acid pyrophosphate.

3. A flame retardant composition comprising an intimate mixture of apolymer of methyl methacrylate and as a flame retardant additive2,3-dibromopropyl methyl acid phosphate.

4. .A flame retardant composition comprising an intimate mixture of apolymer of methyl methacrylate and as a flame retardant additive (a) amixture of mono-2,3-dibromopropyl acid phosphate and 2,3-dibromopropylmethyl acid phosphate, said mixture comprising at least about 30 molepercent of said additive with the balance of said additive comprising(b) a mixture of monoand dimethyl acid phosphate andbis(2,3-dibromopropyl) acid phosphate; said additive comprising theproduct resulting from the reaction between about 1.8 moles of methanol,about 1.2 moles of 2,3- dibromopropanol and about 1.0 mole of phosphoruspentoxide.

5. A flame retardant composition comprising an intimate mixture of apolymer of methyl methacrylate and as a flame retardant additive amixture of: (a) mono-2,3-dibromopropyl acid phosphate (b)2,3-dibr0mopropyl methyl acid phosphate (c) mono-methyl acid phosphateand (d) bis(2,3- dibromopropyl) acid phosphate said mixture being theproduct resulting from the reaction between about 1.0 mole of methanol,about 2.0 moles of 2,3-dibromopropanol and about 1.0 mole of phosphoruspentoxide.

2. A flame retardant composition comprising an intimate mixture of apolymer of methyl methacrylate and as a flame retardant additivebis(2,3-dibromopropyl acid pyrophosphate.
 3. A flame retardantcomposition comprising an intimate mixture of a polymer of methylmethacrylate and as a flame retardant additive 2,3-dibromopropyl methylacid phosphate.
 4. A flame retardant composition comprising an intimatemixture of a polymer of methyl methacrylate and as a flame retardantadditive (a) a mixture of mono-2,3-dibromopropyl acid phosphate and2,3-dibromopropyl methyl acid phosphate, said mixture comprising atleast about 30 mole percent of said additive with the balance of saidadditive comprising (b) a mixture of mono-and dimethyl acid phosphateand bis(2,3-dibromopropyl) acid phosphate; said additive comprising theproduct resulting from the reaction between about 1.8 moles of methanol,about 1.2 moles of 2,3-dibromopropanol and about 1.0 mole of phosphoruspentoxide.
 5. A flame retardant composition comprising an intimatemixture of a polymer of methyl methacrylate and as a flame retardantadditive a mixture of: (a) mono-2,3-dibromopropyl acid phosphate (b)2,3-dibromopropyl methyl acid phosphate (c) mono-methyl acid phosphateand (d) bis(2,3-dibromopropyl) acid phosphate said mixture being theproduct resulting from the reaction between about 1.0 mole of methanol,about 2.0 moles of 2,3-dibromopropanol and about 1.0 mole of phosphoruspentoxide.